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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  TCP/IP Routing Protocols (Gateway Protocols)                 9  TCP/IP Interior Routing Protocols (RIP, OSPF, GGP, HELLO, IGRP, EIGRP)                      9  Other Interior Routing Protocols

TCP/IP Gateway-to-Gateway Protocol (GGP) 1 2 Interior Gateway Routing Protocol (IGRP)

Issues with Using Delay as a Metric

In theory, using delay calculations should result in more efficient route selection than simply using hop count, but at the cost of more complexity than a hop-count algorithm. This makes HELLO very interesting indeed, especially for a protocol that is over 20 years old. However, since the latency of a link is often unrelated to its bandwidth, using time delay as a link metric may lead to spurious results.

Furthermore, it is normal for the delay on any link to vary over time; in the case where there are two routes that are similar in cost, fluctuations in the delay for each route could result in rapid changes between routes (a phenomenon sometimes called route flapping). Adjustments are needed to the basic overview of the operation of the HELLO protocol above, to avoid these sorts of problems.

Like other early routing protocols, HELLO includes nothing fancy like authentication and so on; these features were not really needed in the early days of the Internet, when the internetworks were small and easily controlled. As the Internet grew, HELLO was eventually replaced by newer routing protocols such as RIP. It is now considered a historical protocol (read: obsolete) and is no longer used.

TCP/IP Gateway-to-Gateway Protocol (GGP) 1 2 Interior Gateway Routing Protocol (IGRP)